In general, in a transmission-type electron microscope, a specimen is irradiated with an electron beam that is focused by condenser lenses. The beam penetrates the specimen, and forms a diffraction pattern of the specimen in the back focal plane of the objective lens. Then, the diffracted electrons are again caused to interfere with each other to produce a magnified image of the specimen. When this magnified image is projected onto a fluorescent screen through projector lenses, a magnified transmission image of the specimen can be observed. When the back focal plane of the objective lens is projected, a magnified diffraction pattern of the specimen can be observed. Where intermediate lenses are mounted between the objective lens and the projector lenses, either a magnified transmission image or a diffraction pattern can be obtained by adjusting the focal length of the intermediate lenses. One conventional method of observing such a magnified transmission image or diffraction pattern (hereinafter referred to as a "transmission electron optical image"), is to place a sheet of photographic film in the focal plane of the projector lenses and to expose the film with the transmission electron optical image. Another conventional means is to use an image intensifier for magnifying the projected transmission electron optical image. (European Patent Application No. 0168838)
However, the sensitivity of photographic film to electron beams is low. In addition, it is laborious to develop the film. Also, the use of an image intensifier introduces the problem that the sharpness of the image is low. Further, the image tends to be distorted.
Recently, a quite sensitive two-dimensional sensor has been proposed. This sensor stores the energy of the electron beam impinging on it. Then, the sensor is exposed to light or heated so that it emits light in proportion to the stored energy.
FIG. 3 is a cross-sectional view of a stimulable phosphor sheet adapted for the above-described two-dimensional sensor, the sheet being stimulated by stimulating rays. The stimulable phosphor sheet, indicated by numeral 1, comprises a support 2 and a stimulable phosphor layer 3 formed on the base 2. The base 2 can be a sheet of polyethylene or plastic film of about 100 to 200 .mu.m thick, a sheet of aluminum of 0.5 to 1 mm thick, a sheet of glass of 1 to 3 mm thick, or the like. The base 2 may or may not be transparent. Where the base is opaque, the light emitted from the stimulable phosphor sheet is detected from the same side as the stimulating rays impinged. Where the base is transparent, the emitted light can be detected from the same side the stimulating rays impinged from the opposite side to the stimulating rays impinged on both sides.
The stimulable phosphor used in stimulable phosphor sheet of the two-dimensional sensor employed in the present invention can be: EQU (Ba.sub.1-x-y, Mg.sub.x, Ca.sub.y) FX:.alpha.Eu.sup.2+ ( 1)
where X is at least one of Cl and Br; x and y satisfy the conditions 0&lt;x +y.ltoreq.0.6 and xy .noteq.xy 0; and .alpha. satisfies the condition 10.sup.-6 .ltoreq..alpha.5.times.10.sup.-2. This is described in Japanese Patent Unexamined Laid Open No. 12143/1980. EQU LnOX : xA (2)
where Ln is at least one selected from the group consisting of La, Y, Gd, and Lu; X is at least one of Cl and Br; A is at least one of Ce and Tb; and x satisfies the condition 0&lt;x&lt;0.1. This is described in U.S. Pat. No. 4,236,078. EQU M.sup..pi. FX.multidot.XA: yLn (3)
where M.sup..pi. is at least one selected from the group consisting of Ba, Ca, Sr, Mg, Zn, and Cd; A is at least one selected from the group consisting of BeO, MgO, CaO, SrO, BaO, ZnO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, In.sub.2 O.sub.3, SiO.sub.2, TlO.sub.2, ZrO.sub.2, GeO.sub.2, SnO.sub.2, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, and ThO.sub.2 ; Ln is at least one selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, HO, Nd, Yb, Er, Sm, and Gd; X is at least one selected from the group consisting of Cl, Br, and I; x satisfies the condition 5.times.10.sup.-5 &lt;.times..ltoreq.0.5; and y satisfies the condition 0&lt;y.ltoreq.0.2. This stimulable phosphor is described in U.S. Pat. No. 4,539,138. EQU BaFX.multidot.xNaX':.alpha.Eu.sup.2+ ( 4)
where each of X and X' is at least one of Cl, Br, and I; x satisfies the condition 0&lt;x.ltoreq.2; and .alpha. satisfies the condition 0&lt;.alpha..ltoreq.0.2. This stimulable phosphor is described in Japanese Patent Unexamined Laid Open No. 56479/1984. EQU M.sup..pi. X.sub.2 .multidot.aM.sup..pi. X'.sub.2 : xEu.sup.2+( 5)
where M.sup..pi. is at least one alkaline-earth metal selected from the group consisting of Ba, Sr, and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br, and I; X and X' are different halogens (X.noteq.X'); .alpha. is a numerical value satisfying the condition 0.1.ltoreq..alpha..ltoreq.10.0; x is a value satisfying the condition 0&lt;x.ltoreq.0.2. This stimulable phosphor is described in Japanese Patent Application No. 84381/1985 (U.S. patent Ser. No. 834,886), and can contain an additive as described in Japanese Patent Application No. 166379/1985 (correspond to European patent application No. 151,494) or 221483/1985 (U.S. Ser. No. 947,631, European Patent Application No. 159,014).
Other usable storage-type fluorescent substances are described in U.S. Pat. Nos. 3,859,527, 4,236,078, 4,239,968, 4,505,989 and Japanese patent application Nos. 116777/1981, 23673/1982, 23675/1982, 69281/1983, 206678/1983 (U.S. Ser. No. 841,044, European Patent Application No. 95741), 27980/1984 (European Patent Application No. 101,030), 47289/1984 (European Patent Application No. 103,302), 752200/1984 (European Patent Application No. 107,192), and 101173/1985. Any one of these storage-type fluorescent substances dispersed in a suitable binder and applied to the support 2 up to a thickness of 1000 microns. If stimulable phosphor layer can sustain itself, it can form stimulable phosphor sheet by itself.
When a stimulable phosphor sheet formed in this way is exposed to an electron beam or other radiation, some of the energy of the radiation is stored in the stimulable phosphor. Subsequently, the sheet is exposed to stimulating rays such as visible light. As a result, the fluorescent substance fluoresces according to the stored energy. Instead of the stimulable phosphor stimulated by light, a thermal phosphor can be used which releases the stored energy when heated after it stores radiation energy. In this case, the thermal phosphor is sulfate, such as Na.sub.2 SO.sub.4, MnSO.sub.4, CaSO.sub.4, SrSO.sub.4, or BaSO.sub.4, to which a trace of at least one of the additives Mn, Dy, and Tm, is added. The thermal phosphor sheet is fabricated in the same manner as the aforementioned stimulable phosphor sheet.
A two-dimensional sensor made of the abovedescribed stimulable phosphor sheet or thermal phosphor sheet is placed in the focal plane of an electron microscope. A transmission electron optical image is stored in this sensor, which is scanned either with stimulating rays such as visible light r with a heat source so that it may emit light. The resulting emitted light is detected photoelectrically. As a result, an electric signal corresponding to the transmission electron optical image can be obtained. The signal derived in this way can be fed to a display unit such as a CRT to make visible the electron optical image, or it can be permanently recorded in the form of hard copy. It is also possible to temporarily store the image signal on a recording medium such as magnetic tape or magnetic disk.
When the aforementioned two-dimensional sensor is used, a black spot is developed at the center of the produced image. This phenomenon does not occur when the conventional photographic film is employed. The occurance of such a phenomenon is now explained. The sensitivity of the two-dimensional sensor is much higher than that of the conventional photographic film. Therefore, the image of the filaments of the electron gun installed on the main optical axis of the electron beam in the upper portion of the electron microscope is recorded, or the electrons emanating from the filaments are scattered by the apertures and the specimen installed on the optical axis, giving rise to other radiation, such as X-rays. Hence, the black spot appears.